1. Field of the Invention
The present invention relates to a memory element including a semiconductor device and a signal processing circuit including a semiconductor device.
2. Description of the Related Art
Transistors using amorphous silicon, polysilicon, microcrystalline silicon, or the like have been used for display devices such as liquid crystal displays conventionally. Nowadays, a technique in which such transistors are utilized for semiconductor integrated circuits is proposed (e.g., see Patent Document 1).
In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material having high mobility equivalent to that of polysilicon or microcrystalline silicon and having uniform element characteristics equivalent to those of amorphous silicon.
The metal oxide is used for a variety of applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. As metal oxides having semiconductor characteristics, for example, there are tungsten oxide, tin oxide, zinc oxide, and the like, and a transistor in which a channel formation region is formed using such a metal oxide having semiconductor characteristics has been known (see Patent Documents 2 to 4).